Convert pnm to dds Online: pnm2dds
You can use Tom's Editor to easily convert images from
Portable aNy Map .PNM to Direct Show Surface
Tom's Editor is a free, online image manipulation and converter program.
Go to Tom's Editor
PNM images are uncompressed 24 bit RGB images. Can cannot store Alpha channel. PNM images come in 2 types: binary and text. Especially popular on Linux.
DDS are files invented by Microsoft to be used in games, mostly as textures. Images can be compressed to save space but the compression methods defined by DDS are all very fast in terms of coding and decoding.